Polymers produced from metallocene catalysts and other single site catalysts generally produce polymers with narrow molecular weight distribution (also designated MWD), commonly characterized by the ratio of weight-averaged/number-averaged molecular weights (Mw/Mn), in well-mixed reactors. For example, the Mw/Mn values for polymers made with metallocene catalyst systems in homogeneous polymerization media are typically close to the statistically expected value of 2.0 (for the description of metallocene, Ziegler-Natta, and other olefin polymerization catalysts see J. Scheirs, W. Kaminsky, Ed., “Metallocene-Based Polyolefins”, Wiley, New York, 2000; E. P. Moore, Jr., Ed, “Polypropylene Handbook”, Hanser, New York, 1996; L. S. Baugh, J. A. M. Canich, Ed., “Stereoselective Polymerization With Single-Site Catalysts”, CRC, New York, 2008). Other catalysts of commercial significance (such as Ziegler-Natta), however, make polymers with much broader MWD. Single site catalysts, such as, for example, metallocenes, also produce narrow composition distribution. The composition distribution is the distribution of monomer composition and/or crystallinity from polymer molecule to molecule within the bulk polymer. For, example, isotactic polypropylenes made with Ziegler-Natta catalysts often have Mw/Mn values as high as 4-6, and a broad crystallinity distribution.
While narrow MWD and narrow CD generally conveyed by metallocene catalysts may be advantageous in some applications, there are certain applications where a narrower MWD and CD are deleterious, for example in melt processing and solid state processing (orientation) of polymers. In particular, in film processing and fiber processing applications, a broader molecular weight distribution provides particular advantages in both melt processability and orientation processability. Prior-art approaches for broadening molecular weight distribution include in-situ approaches, such as utilizing series or parallel reactors and also the use of mixed catalysts. Melt-blending in an extruder of polymer components with differing distributions of molecular weight to provide for broadened molecular weight distribution is also widely practiced. These prior-art methods for broadening molecular weight distribution add both process complexity and cost.
Hence, there is a need for alternative approaches to tailoring the MWD and CD of polymers produced from single site type catalysts so as to produce polymers with a broader molecular weight distribution in a continuous homogeneous polymerization process.